Batch processing GUI

[svandajan]

Added simple GUI for batch signing files.

This is still very basic, in the future, I would like to add ability to set different options for files (specifically position of visible signature) and ability to store options into profiles (and maybe some dynamic settings, like adding visible signature to the last page of PDF file).

[kwart]

Thank you for this work, @svandajan. Since 3.0.0 the default UI is JavaFX (see jsignpdf/src/main/java/net/sf/jsignpdf/fx/), and Swing is fallback only, so merging a Swing-side batch window would land on the deprecated code path. Issue #30 tracks batch signing in the GUI and is still open — would you be interested in porting this to JavaFX? I'll keep this PR open as a reference for the design. If there's no capacity to port, I'll close it in a few weeks and link back from #30.